The present invention relates to the compression of digital data, and more particularly to a method and apparatus for processing digitized still picture signals for transmission or storage in a compressed form.
The communication and storage of still pictures, which may include graphics and/or text, has become commonplace with the advent of techniques for digitizing picture signals. Applications for such technology include facsimile, medical diagnostics, the publication of books and periodicals on compact disc read-only memories (CD-ROM), and the like.
A substantial amount of digital data must be stored (e.g., on a CD-ROM) or transmitted (e.g., via satellite) in any digital picture communication system. In order to most efficiently use the available storage capacity or radio frequency spectrum, it is advantageous to compress the digital picture signals to minimize the amount of data that must be stored or transmitted. Video compression techniques are known for efficiently transmitting digital video signals over conventional communication channels, for example in the transmission of video television signals via satellite or cable. Such techniques use compression algorithms that take advantage of the correlation among adjacent pixels in order to derive a more efficient representation of the important information in a video signal.
The most powerful compression systems not only take advantage of spatial correlation, but can also utilize similarities among adjacent frames of a television picture to further compact the data. In such "interframe" systems, differential encoding is used to transmit only the difference between an actual frame and a prediction of the actual frame. The prediction is based on information derived from a previous frame of the same video sequence. Examples of such systems can be found in Ninomiya and Ohtsuka, "A Motion-Compensated Interframe Coding Scheme for Television Pictures", IEEE Transactions on Communications, Vol. COM-30, No. 1, Jan. 1982 and Ericsson, "Fixed and Adaptive Predictors for Hybrid Predictive/Transform Coding", IEEE Transactions on Communications, Vol. COM-33, No. 12, Dec. 1985, both incorporated herein by reference.
The techniques described in the aforementioned articles relate to "motion compensation" techniques that are applicable to successive frames of a television picture. Such techniques do not address the compression of digitized still pictures. In applications where a succession of different still pictures are stored or transmitted, there is little or no correlation between the content of the successive images. In such applications, motion compensation techniques that rely on the prediction of current video data from the video content of a previous frame would most likely produce error signals that contain more, rather than less data. This would defeat the benefits of signal compression.
It would be advantageous to provide a method and apparatus for compressing digitized still pictures in order to reduce the volume of data that must be communicated to a data receiver in order to store or transmit the picture information. It would be further advantageous to provide such a system that would work in combination with known motion compensation systems in the transmission of digital television signals. The present invention provides a method and apparatus enjoying the aforementioned advantages.